The invention relates to a hydraulic circuit arrangement for setting the displacement of a pump, with a device for pressure limiting control, according to the features of Claim 1, and to a method for pressure control by controlling the displacement according to the features of Claim 7.
Pumps which are often used together with hydraulic motors as hydraulic transmissions in hydrostatic travel drives have a stroking mechanism, by means of which the displacement of the machine can be varied. For this purpose, the pumps possess, for example, an angularly adjustable swashplate on which the pistons located in a rotating cylinder block are supported. The displacement generated by the pistons is set indirectly via the pivot angle of the swashplate. The latter is adjusted by means of a servo system, for example a piston movable in a cylinder, the movement of which is regulated via a control-valve unit, called a control, which can be activated electrically via magnets or manually. By means of the control, one of the two servo sides is acted upon alternately with pressure. At the same time, the other side is relieved towards the tank via suitably dimensioned orifices. The side connected to the charge pressure from the control is designated as the “active servo side”. The “passive servo side” is the side connected to the tank via the control.
Pressure limiting operation is set by means of pressure-limiting valves, which are connected to the working lines and connect the servo lines to the working lines of the pump in such a way that, for example during normal pump operation, the pressure limiting valve opens the high-pressure side of the pump towards the passive servo side when the pressure setting of the pressure limiting valve is reached. Although pump operation is by far the most predominant type of operation, the motor operation of the pump, dependent on the driving state of the vehicle, for example, an overrun operation downhill, also has to be taken into account. With the control being activated identically, in motor operation, the high-pressure sides alternate. In this case, different effects are achieved by means of pressure limiting regulation. When the unit is in pump operation, corresponding to the driving states of “normal drive” or “acceleration”, when the pressure limit is reached the pivot angle of the unit is reduced and the high pressure is regulated to the set value. In motor operation, that is to say in “braking or overrun operation” of the vehicle, by contrast, the pivot angle is increased when the pressure setting is reached, so that the displacement generated, in this case by the hydraulic motor, can be absorbed.
The use of the abovementioned pressure limiting valves results, as a function of time, depending on the orifices present in the inflows and return flows to and from the servo system, in a considerable pressure load and in adverse reaction times, in particular too long pivot-back times in which the pressure load persists for a correspondingly long time.
The object of the invention is therefore to provide a system for setting the displacement of a pump with improved pressure limiting pressure regulation.